This invention relates to a rolling contact bearing coated with an insulating film to improve resistance to electrolytic corrosion.
In order to prevent electrolytic corrosion which may occur between bearing rings and rolling elements, it is a known practice to provide insulating coatings or films on the surfaces of bearing rings of a rolling contact bearing such as a ball bearing or a roller bearing, thus preventing a current from flowing to the bearing rings from outside. (e.g. Japanese Utility Model Application 63-165244)
Since such insulating coatings are formed on a bearing adapted to be mounted between a rotary shaft and a housing with a predetermined interference, they are required to be sufficiently low in water absorption so that they will not suffer dimensional changes and also to have a high creep resistance enough to prevent and decrease in the thickness of the insulating films and thus to keep the interference of the bearing unchanged.
Also, such insulating films are required to be sufficiently heat-resistant so that they will not suffer deformation or change in properties even if the bearing is subjected to high temperatures while being rotated at high speed. Further, their moldability has to be sufficiently high so that they can be mass-produced at low cost.
Engineering plastics are now drawing much attention as materials for functional parts of mechanical or electrical parts. Among such plastics, polyamide 66 (hereinafter referred to as PA66), polyamide 6 (PA6), polybutylene terephthalate (PBT) and polyethylene terephthalate (PET) are known as materials having high insulation properties and thus suitable for use as insulating films. But, an insulating film made of such materials has a problem in that they do not meet the requirements completely.
Namely, though it is possible to increase the creep resistance of PA66 and PA6 by adding glass fiber as a reinforcing material, these materials have a problem in that their water absorption coefficient is so high that their dimensions tend to change remarkably, Also, their insulation properties tend to change with temperature, so that they show unstable insulating performance.
On the other hand, PBT and PET are low in water absorption and thus are less likely to suffer dimensional changes and decline in insulation properties. But their heat resistance and creep resistance are not sufficient as materials for insulating films.
Namely, while an insulating film to be formed on a bearing adapted for high-speed operation is required to withstand a temperature of about 150.degree. C., the tensile strength of PBT and PET reduces to half at a lower temperature than 150.degree. C. Thus, no sufficient strength is obtainable at high temperatures.
If glass fiber is added to these materials to increase the creep resistance, their moldability will worsen markedly (this is especially true with PET), Thus, it is difficult to form these materials into a thin insulating film by injection molding.
An object of the present invention is to provide a bearing coated with insulating films which can protect the bearing against electrolytic corrosion and have excellent insulating properties.